Model doesn't change after sculpting
It's the Multires modifier. It does not like changes on the underlying mesh and this can result in strange spikes or something like you have.
You can try to fix it with the Multires Displacement Eraser brush in Sculpt mode (requires Blender 2.9+). I am not sure if that helps but it's worth a try.
If it doesn't help, you can fix it with the 'cut-off' method:

In Edit mode, select the bad faces of the low poly mesh and separate them with P, Separate &gt; Selection
Switch to Object mode, select the new object with the bad faces and remove the Multires modifier. The faces should now look clean and 'low-poly'.
Select now both objects (the cut-off object and the original with the hole). Make sure you select the original last (=active object, orange selection color. This way it will keep the Multires modifier.).
Join the objects (Ctrl+J). (If the Multires modifier disappeared then you selected them in the wrong order, Ctrl+Z to undo ;-)
In Edit mode, select everything with A, and merge the loose meshes with M Merge &gt; By Distance

That's it. The mesh should look nice again, except for the &quot;low-poly spot&quot;. You just must add the missing details (Sculpt mode) for the spot.